Methods and equipment for induction of circulating flow in aquaculture enclosures are known in the art. Circular tanks are most commonly used, due to their inherent structural strength, and because they can maintain a characteristic rotating flow, against which finfish are induced to swim. Swimming exercise is believed to promote weight gain and feed conversion efficiency in some species of finfish.
In one common design, water is introduced into a circular rearing tank at the perimeter, in a tangential direction, so as to impart angular momentum to the fluid flow, and is withdrawn from the central axis of the tank through a standpipe or floor drain. The primary flow in this design follows a spiral path from the perimeter toward the center. It is also known that such azimuthal flow in circular tanks induces a secondary, toroidal flow by a mechanism known as the ‘teacup effect’; centrifugal pressure exerted on fluid at the rotating free surface boundary is not balanced by the slower boundary layer-influenced flow adjacent to the floor of the tank. The pressure imbalance induces flow radially outward along the free surface, down the vertical tank wall, and radially inward across the floor, back to the central axis, where fluid is displaced vertically upward creating a hydraulic circuit. The teacup effect is responsible for the self-cleaning property of circular tanks, whereby settle-able solid debris, including fecal matter, uneaten feed pellets, and moribund fish, are swept in a spiral path toward the center of the floor and out through a drain.
In a variant of this design, the majority of the flow exiting the tank is drawn from an overflow weir at the upper side wall, while the solids exit through the center drain with the remainder of the flow. This configuration concentrates the solid waste in a relatively small proportion of the flow stream and facilitates de-watering and treatment steps of recirculating aquaculture systems.
U.S. Pat. Nos. 3,653,358 and 3,698,359 to Fremont describe a watertight liner suspended from a floatation collar of flexibly linked, foam-filled floats and provided with inlet and outlet pipes, and oxygen spargers to continuously oxygenate the enclosed water. Flow pattern is from one end of an elongate enclosure to the other, as is the case with land-based ‘raceway’ enclosures, or is not specified.
U.S. Pat. No. 4,211,183 to Hoult describes a land-based recirculating aquaculture system with centrally located upwelling pump and central drain with integral bio-filter. In one implementation the bio-filter support follows a spiral path, but no mention is made of the circulation pattern within the rearing volume of the tank, or particularly of the effect of feeding circulation from the central top surface of the water volume.
U.S. Pat. No. 4,798,168 to Vadseth describes a floating closed-containment aquaculture enclosure with an externally mounted vertical pump duct drawing water from depth, discharging horizontally tangentially into the perimeter of the floating enclosure. Water follows a spiral path with induced poloidal component, and exits through a center standpipe drain.
U.S. Pat. No. 6,443,100 to Brenton further describes the flow pattern within floating closed-containment enclosures, and claims a design of standpipe drain for such rearing enclosures that extracts clear effluent and solids through separate pipes.
None of the previously described methods specifically address the changes in intrinsic fluid behavior as aquaculture enclosures are scaled from volumes in the order of 100 cubic meters typical of land-based culture systems to volumes in the order of 10,000 cubic meters required for large scale grow-out operations typical in the modern culture of salmonids and tunas. Such tanks may have diameters of up to 40 meters, and depths to 15 meters. At this scale, two practical difficulties arise with the azimuthal flow pattern and with the teacup effect. Firstly, tangential velocity at the perimeter of the tank produced by the flow volume necessary to exchange the large volume of enclosed water volume in the time required (on the order of one hour) is higher than the preferred swimming speed of the cultured fish, particularly in the early life stages. Secondly, the teacup effect becomes less significant as the Reynolds number of the flow increases. At large scale, turbulence and momentum predominate, while viscosity, which is responsible for the boundary layer which drives the toroidal flow component, is less influential in determining the overall behavior of the flow. In practice, solids are seen to build up on the floor of the tank, the central vortex drifts from the axis or bifurcates, and in extreme cases multiple concentric toroidal vortices develop, with upwelling zones re-suspending solids. The toroidal flow of water within an enclosure will tend to concentrate particles in the center of the enclosure. This is known as the “tea leaf paradox” or “tea leaf effect”, a type of momentum coupling effect explained by Albert Einstein in The Cause of the Formation of Meanders in the Courses of Rivers and of the So-Called Baer's Law, Die Naturwissenschaften, Vol. 14, 1926 essentially as follows. Stirring the liquid makes it spin around the cup. In order to maintain this curved path, a centripetal force in towards the center is needed (similar to the tension in a string when spinning a bucket over your head). This is accomplished by a pressure gradient outward (higher pressure outside than inside). However, near the bottom and outer edges the liquid is slowed by the friction against the cup. There the centripetal force is weaker and cannot overcome the pressure gradient, so these pressure differences become more important for the water flow. This is called a boundary layer. Because of inertia, the pressure is higher along the rim than in the middle. If all the liquid rotated as a solid body, the inward (centripetal) force would match the outward (inertial) three from the rotation and there would be no inward or outward movement. In a teacup, where the rotation is slower at the bottom, the pressure gradient takes over and creates an inward flow along the bottom. Higher up, the liquid flows outward instead. This secondary flow travels inward along the bottom bringing the leaves to the center, then up, out and down near the rim. The leaves are too heavy to lift upwards, so they stay in the middle. Combined with the primary rotational flow, the leaves will spiral inward along the bottom. The primary rotational flow is toroidal, and the secondary flow is poloidal. As defined by the Oxford English Dictionary (OED), the “toroidal” direction is the long way around a torus (a donut-shape), and the “poloidal” direction is the short way around the torus. Prior art systems in the field of the invention drive the toroidal flow axis by injecting flow radially at the outside wall, and the secondary poloidal axis flow is induced by teacup effect.